Does this idiom indicate the negative side or positive side? Example: what you have seen about his character is just the tip of the iceberg...
Does this example show whether he has so much good quality hidden or he has so much bad quality hidden...
According to Oxford dictionary "the tip of the iceberg" refers to the perceptible part of a much larger "problem or situation" so I'd say the connotation is negative.
For a positive expression you could perhaps use "it has only scratched the surface"
This is fascinating.
Like Oxford, Cambridge says:
a small, noticeable part of a problem, the total size of which is really much greater
But per Merriam-Webster:
: the earliest, most obvious, or most superficial manifestation of some phenomenon
And The Free Dictionary:
Only a small, often unrepresentative portion of something much larger or more complex that cannot yet be seen or understood.
In the latter definitions, it has no normative meaning. It can be good, bad, or neutral.
Personally, I've heard it used to express something positive (or neutral) just as much as I've heard it used to express a negative.
For instance (to make up something that sounds natural to me):
Just look at some of the benefits. And they're only the tip of the iceberg.
I've always taken it to mean "there's more to this than meets the eye." While it's true that an iceberg sank The Titanic, that's not what I think of when I think of icebergs. (If I'm not thinking of the Arctic or Antarctic in general, I have a greater association with penguins than I do with the famous ship . . .)
Given this, I would assume nothing without context.
As for this specific question, whatever the quality is that we've already seen about his character (it has not been stated what we've already seen), it will determine the type of further, hidden qualities.